The present invention relates to a method of detecting ink concentration in an ink jet recording apparatus, an ink concentration detecting apparatus with which the ink concentration detecting method is implemented, and an ink jet recording apparatus using the ink concentration detecting apparatus.
As for a recording method in which ink droplets each containing colorant particles are ejected onto a recording medium to record an image thereon, there is for example an electrostatic ink jet recording system in which ink containing charged colorant particle components is used, and ejection of the ink droplets is controlled by utilizing an electrostatic force generated by applying predetermined voltages (drive voltages) to ejection electrodes (drive electrodes) of an ink jet head in accordance with image data to thereby record an image corresponding to the image data on a recording medium.
Known as an example of a recording apparatus using the electrostatic ink jet recording system is an ink jet recording apparatus disclosed in JP 10-138493 A.
FIG. 9 is a schematic view showing an ink jet head used in the electrostatic ink jet recording apparatus disclosed in JP 10-138493 A.
An ink jet head 200 includes a head substrate 202, an ink guide 204, an insulating substrate 206, a control electrode 208, a D.C. bias voltage source 210, and a signal voltage source (pulse voltage source) 212.
A through hole 214 through which ink is to be ejected is formed so as to extend perfectly through the insulating substrate 206. The head substrate 202 is provided in a position apart from the insulating substrate 206 by a predetermined distance, and ink guides 204 are disposed in positions on the head substrate 202 corresponding to the through holes 214. The ink guide 204 extends perfectly through the through hole 214 so as for its tip portion 204a to project upwardly and beyond a surface of the insulating substrate 206 on a side of a recording medium P.
In addition, the recording medium P is disposed in a position where the recording medium P faces the tip portion of the ink guide 204. A counter electrode 218 serving as a platen as well for guiding the recording medium P is disposed in a rear surface of the recording medium P opposite to the head substrate 202. Also, the counter electrode 218 is grounded.
The head substrate 202 is disposed at a predetermined distance from the insulating substrate 206. Thus, an ink flow path 216 is defined between the head substrate 202 and the insulating substrate 206.
The ink containing colorant particles which are charged to the same polarity as that of a voltage applied to the control electrode 208 is made to circulate through the ink flow path 216 from the right-hand side to the left-hand side in FIG. 9, for example, by a circulation mechanism for ink (not shown). Thus, the ink is supplied to the corresponding ones of the through holes 214.
The control electrode 208 is provided in a ring-like shape on the surface of the insulating substrate 206 on the side of the recording medium P so as to surround the periphery of the through hole 212. In addition, the control electrode 208 is connected to the pulse voltage source 212 for generating a pulse voltage in accordance with image data. The pulse voltage source 212 is grounded through the D.C. bias voltage source 210.
In such an electrostatic ink jet recording apparatus, in a state where no pulse voltage is applied to the control electrode 208, the Coulomb attraction between the bias voltage and the charged toner particles (colorant particles) in the ink, the viscosity of the ink (carrier liquid), the surface tension, the resiliencies between the charged particles, the fluid pressure when the ink is supplied, and the like operate in conjunction with one another. Thus, the balance is obtained among these factors in a state where as shown in FIG. 4, the ink has a meniscus shape of slightly rising from the through hole 214.
In addition, the colorant particles migrate to the meniscus surface due to the Coulomb attraction or the like, i.e., there is provided a state where the ink is concentrated on the meniscus surface.
When the voltage is applied to the control electrode 208 (ejection is ON), the bias voltage is superposed on the drive voltage so that the ink is attracted towards a side of the recording medium P to form a nearly conical shape, i.e., a so-called Taylor cone.
When a time elapses after application of the voltage to the control electrode, the balance between the Coulomb attraction acting on the colorant particles and the surface tension of the carrier liquid is broken. As a result, there is formed a slender ink liquid column having a diameter of about several microns to several tens of microns which is called a thread. When a time further elapses, a tip portion of the thread is divided, and as a result, droplets of the ink are ejected to fly towards the recording medium P by the electrostatic attraction force.
In the electrostatic ink jet recording process, normally, a modulated pulse voltage is applied to the corresponding ones of the control electrodes 208 to turn ON/OFF the corresponding ones of the control electrodes 208 to eject ink droplets. Thus, the ink droplets are ejected on demand in correspondence to an image to be recorded.
Hence, the division of the thread is caused at a frequency much higher than the drive frequency for the pulse voltage used to eject the ink droplets. That is, the division of the thread is continuously caused multiple times for a time period required to apply a pulse voltage to the corresponding ones of the control electrodes once. Consequently, one dot on the recording medium P is formed with a plurality of minute droplets which were separately ejected.
In such an ink jet recording apparatus, if the concentration (ink concentration) of the colorant particles contained in the ink changes, the concentration property of the colorant particles in a through hole (ejection hole), a size of the formed thread, an ejection frequency of the ink droplets, and the like change accordingly. Hence, the control for the concentration of the colorant particles contained in the ink to be supplied to the ejection portion is very important in terms of the control for the ejection of the ink droplets.
Then, JP 2,834,100 B discloses an electrostatic ink jet recording apparatus having ink containing charged toner particles (charged colorant particles), ejection electrodes for ejecting the toner particles in the ink from an ejection opening portion towards a recording medium, and a counter electrode provided so as to be opposite to the ejection opening portion through the recording medium, the electrostatic ink jet recording apparatus including means for detecting an amount of colorant particles in an ink flow using a magnetic sensor, or a light emitting element and a light receiving element, means for judging an amount of detected toner particles, and means for supplying the toner particles to the ink flow based on the judgment results.
In such a manner, the concentration of the toner particles flowing in the ink flow is measured, and the ink is added based on the measurement results, thereby controlling the ink concentration.
However, in the electrostatic ink jet recording apparatus disclosed in JP 2,834,100 B, the detection means is contaminated as the detection means is used, and thus the contamination component becomes noises to exert an influence on the concentration of the toner particles (hereinafter be referred to as “ink concentration”) detected by the detection means. Moreover, a degree of contamination increases along with the use of the detection means.
For this reason, there is encountered a problem in that when the electrostatic ink jet recording apparatus is used for a long time, a difference occurs between the detected ink concentration and the actual ink concentration in the ink flow, and hence it becomes impossible to detect the precise ink concentration. In addition, this difference normally becomes larger with a lapse of time.
Moreover, since the ink concentration cannot be precisely detected, the ink concentration cannot be precisely controlled accordingly. Hence, there is also encountered a problem in that the ink of a suitable concentration cannot be supplied to the ink jet head, and as a result a high-quality image cannot be stably recorded.